The present invention relates to improvements of a method of and an apparatus for wire electrical discharge machining for machining a workpiece on the basis of electric discharge energy by supplying working electric power across a gap between a wire electrode and the workpiece.
Electrical discharge machining has established a solid position as a machining technology for such as dies and molds, and has been extensively used in the fields of the automobile industry, the household electrical appliance industry, the semiconductor industry, and the like.
FIG. 7 is an explanatory diagram of the mechanism of electrical discharge machining. In the drawing, reference numeral 1 denotes an electrode; 2, a workpiece; 3, an arc column; 4, a working liquid; and 5, machining debris produced in electrical discharge machining. Removal machining based on the electric discharge in the workpiece 2 progresses while repeating the cycle (corresponding to steps (a) to (e) in FIG. 7) of the following steps (a) to (e): Namely, these steps are (a) the formation of the arc column 3 due to the generation of electrical discharge, (b) the local fusion and the vaporization of the working liquid 4 due to the thermal energy of electric discharge, (c) the generation of an explosive force of vaporization of the working liquid 4, (d) the scattering of a fused portion (machining debris 5), and (e) cooling, solidification, and recovery of insulation between the electrodes due to the working liquid.
This invention concerns wire electrical discharge machining which is used in boring, cutting, and the like in electrical discharge machining. In particular, there has been a growing demand for higher precision in the wire electrical discharge machining, and high machining accuracy on the order of 1 to 2 xcexcm or thereabouts has come to be required in the machining of high-precision dies and molds used in the semiconductor industry and the like.
FIG. 8 is an explanatory diagram illustrating an example of the machining process of wire electrical discharge machining. In the drawing, reference numeral 1a denotes a wire electrode; 2, the workpiece; 4a, a working liquid which is, for example, water; and 6, an initial hole. The part (a) of FIG. 8 shows the state of a first cut which is rough machining, the part (b) of FIG. 8 shows the state of a second cut which is semi-finish machining after rough finishing, and the part (c) of FIG. 8 shows the state of a third cut which is final finish machining.
The example of the machining of the first cut in the part (a) of FIG. 8 shows machining in which the wire electrode 1a is passed through the initial hole 6, and the workpiece 2 is bored. In the case of such a first cut, since the surface roughness and accuracy are finished in subsequent machining, very strict surface roughness and accuracy are not required so much, and it is important to increase the machining speed, in particular, so as to improve productivity. In wire electrical discharge machining, in order to increase the machining speed, the injection of the working liquid 4a to the gap between the wire electrode 1a and the workpiece 2 is practiced to efficiently discharge the machining debris from the gap. In addition, in order to eliminate the unevenness of the application of the working liquid 4a to the gap and prevent the disconnection of the wire electrode 1a, a method is adopted in which the working liquid 4a stored in an unillusirated working tank and the workpiece 2 is immersed in it. Thus a working-liquid supplying section is used for supplying the working liquid to the gap between the wire electrode 1a and the workpiece 2.
In the above-described conventional wire electrical discharge machining, the machining after the first cut (the part (a) in FIG. 8), such as the second cut (the part (b) in FIG. 8) and the third cut (the part (c) in FIG. 8), is also performed in the working liquid 4a. 
When a voltage is applied across the gap between the wire electrode 1a and the workpiece 2, a force acts in which the positive polarity and the negative polarity are attracted toward each other, so that the wire electrode 1a having small rigidity is pulled toward the workpiece 2 side by this electrostatic force. This causes the vibration of the wire electrode 1a, so that there has been a problem in that high-accuracy machining is made difficult due to such vibration.
In addition, in a state in which the explosive force of vaporization of the working liquid has been generated due to the discharge energy (e.g., the part (c) in FIG. 7), a large force acts on the wire electrode 1a in a direction opposite to that of the workpiece 2 due to the explosive force of vaporization of the working liquid, so that vibrations occur. There has been a problem in that irregularities occur in the shape of the workpiece 2 due to such vibrations, which leads to the deterioration of the accuracy.
Further, in the semiconductor industry and the like, which are the fields of application of wire electrical discharge machining, in the machining of such as a die for IC leadframes, applications are increasing in which extremely high accuracy and very smooth surface roughness are required for a workpiece whose form accuracy is 1 xcexcm and whose surface roughness is 1 xcexcm Rmax or less. In such uses, in particular, the above-described problem ascribable to the vibration and the like of the wire electrode has been noticeable.
As a measure for overcoming such problems of wire electrical discharge machining in a liquid, a technique concerning aerial wire electrical discharge machining has been disclosed in which wire electrical discharge machining is performed in the atmosphere without a working liquid interposed in the gap between the wire electrode and the workpiece (Adachi, Tokyo University of Agriculture and Technology, et al.: xe2x80x9cAttaining High Precision in Second Cuts by Aerial EDM,xe2x80x9d Die and Mold Technology, Vol. 14, No. 7, 1999, p. 154, The Nikkan Kogyo Shimbun, Ltd.). In this technique, it is disclosed that accuracy in the straightness of cut surfaces of workpieces can be improved by wire electrical discharge machining in the atmosphere. However, although its significance from the perspective of the attainment of high precision is large as compared with the ordinary wire electrical discharge machining in a working liquid, it is impossible to make use of the cooling effect by means of the working liquid (e.g., the part (e) of FIG. 7) in a case where wire electrical discharge machining is performed in such an atmosphere or wire electrical discharge machining is performed in a mist. Therefore, it is impossible to sufficiently cool the gap between the wire electrode and the workpiece, and machining progresses in the state in which thermal strains have occurred in the workpiece due to the heat generated by electric discharge. Hence there has been a problem in that this technique cannot cope with applications in which extremely high accuracy and very smooth surface roughness are required for workpieces such as those mentioned above.
This invention has been devised to overcome the above-described problems, and its object is to provide a method of and an apparatus for wire electrical discharge machining which are suitable for attainment of high accuracy and high quality in wire electrical discharge machining.
The method of wire electrical discharge machining in accordance with this invention is a method of wire electrical discharge machining for machining a workpiece by generating electric discharge in a gap between a wire electrode and the workpiece, wherein machining is performed by combining at least two kinds of machining among three kinds of machining including machining in a working liquid, machining in a mist, and machining in a gas, and in a process of machining in the mist or the gas, machining is performed while cooling the workpiece.
The wire electrical discharge machining apparatus in accordance with this invention is a wire electrical discharge machining apparatus for machining a workpiece by supplying electric discharge to a gap between a wire electrode and the workpiece by machining-electric-power supplying section and by relatively moving the wire electrode and the workpiece by positioning sections, the wire electrical discharge machining apparatus comprising: a working-fluid supplying section for supplying at least two working fluids of working-liquid supplying section for supplying a working liquid to the gap, a mist supplying section for supplying a mist to the gap, and a gas supplying section for supplying a gas to the gap; and cooling-fluid supplying section for supplying a cooling fluid for cooling the workpiece.
In addition, the wire electrical discharge machining apparatus in accordance with this invention is a wire electrical discharge machining apparatus for machining a workpiece by supplying electric discharge energy to a gap between a wire electrode and the workpiece by machining-electric-power supplying section and by relatively moving the wire electrode and the workpiece by positioning sections, the wire electrical discharge machining apparatus comprising: working-fluid supplying section for supplying at least two working fluids of working-liquid supplying section for supplying a working liquid to the gap, mist supplying section for supplying a mist to the gap, and gas supplying section for supplying a gas to the gap; cooling-fluid supplying section for supplying a cooling fluid for cooling the workpiece; and a controlling section for effecting a changeover among the supply of the working liquid to the gap by the working-liquid supplying section, the supply of the mist to the gap by the mist supplying section, and the supply of the gas to the gap by the gas supplying section, and for controlling the supply of the cooling fluid to the workpiece by the cooling-fluid supplying section during the supplying of the mist to the gap by the mist supplying section or during the supply of the gas to the gap by the gas supplying section, in accordance with productivity, the accuracy required for the workpiece, and the like.
In addition, in the wire electrical discharge machining apparatus in accordance with this invention, the working-fluid supplying section supplies the working fluid to the gap along the wire electrode, and the cooling-fluid supplying section injects the cooling fluid from outside the working fluid toward the workpiece.
In addition, the wire electrical discharge machining apparatus in accordance with this invention has a fluid supplying section in which the working-fluid supplying section and the cooling-fluid supplying section are combined and are formed integrally.
In addition, in the wire electrical discharge machining apparatus in accordance with this invention, the working-liquid supplying means supplies the working liquid in a working liquid tank to the gap by means of a pump, the gas supplying means supplies the gas such as air to the gap after pressurizing the gas by a compressor, and the mist supplying means produces a mist by mixing the working liquid supplied by the working-liquid supplying means and the gas supplied by the gas supplying means and supplies the mist to the gap.
In addition, in the wire electrical discharge machining apparatus in accordance with this invention, the working-liquid supplying section supplies the working liquid in a working liquid tank to the gap by means of a pump, the gas supplying section supplies the gas such as air to the gap after pressurizing the gas by a compressor, and the mist supplying section produces a mist by mixing the working liquid supplied by the working-liquid supplying section and the gas supplied by the gas supplying section and supplies the mist to the gap.
In addition, in the wire electrical discharge machining apparatus in accordance with this invention, the cooling-fluid supplying section pressurizes the working liquid in the working liquid tank by the pump and supplies it to the gap.
In addition, the wire electrical discharge machining apparatus in accordance with this invention further comprises: a cooling section for cooling the gas pressurized by the compressor.
In addition, in the wire electrical discharge machining apparatus in accordance with this invention, the cooling section is formed by using a heat exchanger disposed in the working liquid tank.